The role of micropinocytosis in capillary permeability is poorly understood. Current knowledge is primarily qualitative and describes it as a transport mechanism for the slow passage of large molecules across capillary walls. Micropinocytosis may play an important regulatory role in the movement of soluble components between the blood and the tissues. The basis for such regulation may be selective transport by micropinocytosis of substances which bind to the surface of endothelial cells or conditions which stimulate or inhibit the rate of micropinocytic ingestion. In order to investigate factors which regulate the rate of micropinocytosis and the role of this method of transport in differential capillary permeability, a system will be developed whereby the dynamics of micropinocytic ingestion can be quantitatively analyzed. Rates of ingestion will be determined by measuring the incorporation of a radiolabeled or fluorescent-tagged protein into isolated capillary endothelial cells from epididymal fat. This system will be amenable to concise definition and will lend itself to manipulation of cellular and physical parameters. Uptake of label will exclusively reflect micropinocytic ingestion. Rapid mixing and sampling techniques will be utilized to administer short discreet pulses of label to the cells. Ingestion rates thus obtained will be used to measure the effect on micropinocytosis of; 1. capillary adenyl cyclase 2. hormones and vasoeffector substances 3. cellulr respiration and protein synthesis 4. mitotic activity 5. temperature and pressure 6. characteristics of the labeled marker.